Packing element comprising at least one recessed portion of specific volume and method of using thereof

ABSTRACT

A packing element for use in a fluidised bed includes a continuous surface with at least one recessed portion. The recessed portion has a volume in the range of 0.2 to 30% of the volume of the packing element itself.

FIELD OF INVENTION

This invention relates to packing elements for use in a fluidised bedfor the treatment of fluid streams.

BACKGROUND INFORMATION

In a typical treatment operation a gas stream is passed upwards througha treatment column, such as a scrubbing tower, and a liquid streampasses downwards through the column countercurrent to the gas. One orboth of the fluid feed streams may contain contaminants, which may begaseous, liquid or solid. On contact between the fluid streams thereoccur mass and energy transfers at a rate affected by a number offactors including the respective stream temperatures and the extent ofcontact. The transfers can be improved by the presence of packingelements in the column and this is especially the case if the gas ispassed upwards at a velocity sufficient to fluidise the packingelements. The so-formed fluidised bed of packing elements representsturbulent conditions which increase the contact between the fluids andtherefore further increase the mass and energy transfer.

Such fluidised bed operations include one or more of absorption,desorption, distillation, heat transfer for heating or cooling,scrubbing, stripping, and particulate or droplet transfer.

Improvements in transfer rates in fluidised beds are much sought after.The aim is to improve energy efficiency of the bed, and thus to increasethe performance of a given column and bed or to maintain a givenperformance while reducing column size and/or bed volume.

Historically the packing elements were typically in the form of hollowspheres. It has been however been shown that non-spherical shapes may bemore energy-efficient. Compared with spherical elements thesealternative shapes provide an enhanced tumbling action which increasesthe turbulence imparted to the fluid streams and thus improves thecontact between them. German patent specification No. 3613151-A showsimprovements in scrubbing efficiencies by using ellipsoidal elements.PCT patent specification WO91/08048 describes elements with their centreof gravity offset from the centre of symmetry (COG/COS offset) toenhance their tumbling motion. U.S. Pat. No. 5,588,986 further teachesthat using COG/COS offset, varying the size, shape and density of theelement, and controlling the velocities of the gas and liquid streamspermits control of the tumble velocity and thus of the transfer rate.

The tumble velocity can be determined by the pressure gradient(quantified as pressure drop ΔP over the settled or pre-fluidised bedheight H_(o)) across elements in the fluidised bed, whereby the higherthe pressure gradient across the elements, the higher the tumblevelocity for a given element. There is however a limit to the tumblevelocity that can be achieved by varying the density and COG/COS offset.

Increases to the pressure gradient by different means, for example byincreasing element density alone, without an increase in tumble velocitycan lead to a loss in energy efficiency characterised by high pressuredrop (ΔP) per number of transfer units (NTU).

SUMMARY OF INVENTION

It is therefore an object of the present invention to increase furtherthe transfer efficiency of a fluidised bed and in particular to increasethe tumble velocity of the bed packing elements.

According to the invention there is provided a packing element for usein a fluidised bed, wherein the packing element has a continuous surfacewith at least one recessed portion, the recessed portion(s) having avolume in the range 0.2 to 30% of the volume of the packing elementitself.

Recessed packing elements according to the invention display an improvedtumbling action over the prior art. The recess, otherwise describedherein as an indentation, acts both to capture down-coming liquid whenthe recess is facing upwards and to capture rising gas when the recessis facing downwards. Both attributes have the effect of increasing therotation or tumbling rate. When the recess is in the upward position thecaptured liquid increases the effective density of the element andraises its centre of gravity, thereby increasing the pressure gradientacross the element and causing it to tumble at an increased velocity.The increase in tumble velocity increases the amount of turbulence inthe gas and liquid streams, which in turn increases eddy mass transferand the inter-fluid contact.

The slug of liquid captured in the recess is released when the elementtumbles, falling into upward-facing recesses in elements at a lowerlevel in the bed. The falling liquid thus contributes to increasingtumble velocity at every point through the bed at which it falls into anelement recess. Similar benefits are achieved by the gas rising throughsuccessive contact with elements at higher levels in the bed.

The presence of the recesses thus improves inter-fluid contactthroughout the bed, with consequent improvements in mass and energytransfer rates. Indeed the use of packing elements according to theinvention has the surprising effect of not increasing the pressuregradient by more than 30% even at liquid to gas volume ratios greaterthan 4 dm³/m³ over the same outline shape without a recess. Liquid holdup in the bed is increased by up to 40% even at liquid to gas ratiosbelow 4 dm³/m³ whereas pressure gradients remain comparable. Theincreased liquid hold up and/or pressure gradient contribute toincreases in tumbling rate or surface renewal rate and interfacialsurface area of up to 40% when compared to a non-indented element ofequivalent size, density and non indented outline shape.

The overall effect is the boosting of mass and heat transfer by up to70% for the same pressure loss or energy consumption or the reduction ofthe static pre-fluidised bed height (H_(o)) and expanded bed height(H_(x)) by up to an additional 40% over existing tumbling fluidisableelements.

The packing element is preferably of hollow plastic material, forexample polypropylene, polyethylene or polyvinyl difluoride (PVDF).Other lightweight solid or hollow materials may alternatively besuitable.

The basic outline shape of the packing element may be of anythree-dimensional form: parallelepipedal, including cubic and cuboid;cylindrical, for example rod-shaped with a circular cross section;conical or frusto-conical; parabaloidal; star-shaped; spherical;ellipsoidal; or similar types, and may be symmetrical or asymmetrical.

Preferably the basic shape of the packing element is ellipsoidal, mostpreferably asymmetric ellipsoidal, for example egg-shaped, ovoidal oracorn-shaped.

Each recess represents a depression or indentation in the surface of thebasic element shape. The shape of each recess per se must be such as tocapture falling liquid or rising gas. A variety of recess shapes satisfythis requirement, including straight-sided shapes and concave dishshapes.

The transverse dimensions of the recess may be equal or unequal, thelatter presenting a recess of elongated shape. One instance of elongatedrecess is an annular recess extending around the circumference of thesurface of the packing element.

Multiple recesses may be present in each packing element but in generalone or two recesses are preferred. The most preferred element accordingto the invention is an element of ellipsoidal shape with either a singlerecess which is located at one end of the longest axis of the shape, ortwo recesses on opposing faces.

The packing element is preferably directly moulded, for example by blowmoulding or injection moulding, into its final shape including therequired recess(es). Alternatively the element can be first formed intoits basic shape and appropriate indentations can then be applied to formthe recess(es). The convenience of manufacturing is an important aspectof the chosen final shape since the elements are required in largenumbers for their fluidised bed duties and must be capable of beingmanufactured in a cost-effective way. The choice of final shape is thusa balance between best operational performance and ease of manufacture.

The shape factor of the recessed element, defined herein as ratiobetween its shortest and longest axes, is preferably in the range 0.3 to0.9 for indented basic outline shapes or in the range 0.3 to 1.0 fornaturally indented shapes. Shapes with a ratio outside these ranges maylead to less efficient transfer rates.

The term “volume” as used herein in relation to the packing element as awhole is the displacement volume of the whole enclosed recessed packingelement, and in relation to the recess(es) per se is the total volume ofthe respective depression(s) compared with the shape the element wouldhave without the depression(s). The volume of the recess(es) can thus beregarded as the “free” or “fill” volume of the element. The recessesmust have a total free or fill volume in the range 0.2 to 30%,preferably 1 to 15%, of the volume of the whole enclosed packingelement.

The invention further provides a method of scrubbing, stripping, heattransfer, particulate transfer or distillation, in a column wherein agas stream and a liquid stream are passed in directions counter-currentto each other through a fluidised bed comprising packing elements asdescribed and claimed herein.

Several factors other than the shape of packing elements must be takeninto account in the performance of a column employing them. Among theseis the size of the elements. In this regard the length of the longestaxis of the elements is preferably within the range 10 to 125 mm. Theeffective density of the recessed element is preferably in the range 100to 500 kg/m³. Elements falling outside these ranges may lead to lessefficient transfer rates.

The transfer rates are also affected by the liquid to gas volume ratioand the gas velocity in the column. The fluidised bed is preferablyoperated at pressure gradients (ΔP/H_(o)) above 900 Pa/m. The gasvelocity range is preferably 0.2 to 10 m/s and the liquid to gas volumeratio is preferably 0.1 to 50 dm³/m³.

BRIEF DESCRIPTION OF DRAWINGS

The invention is further described by way of non-limiting example withreference to the following figures:

FIG. 1 shows a cross-sectional view of a recessed packing elementaccording to an exemplary embodiment of the present invention shownwithin a scrubbing tower;

FIG. 2 shows a cross-sectional view of a recessed packing element of thetype illustrated in FIG. 1 and shown in various positions duringtumbling;

FIG. 3 shows a cross-sectional view of a second type of recessed packingelement according to the present invention, in this instance with tworecesses;

FIG. 4 illustrates a graph showing the performance of various packingelements, some according to the present invention; and

FIG. 5 illustrates a graph showing the energy efficacy of packingelements shown in FIG. 3.

DETAILED DESCRIPTION

FIG. 1 shows a cross-sectional view of a packing element according tothe invention in a scrubbing column. The packing element 1, of mouldedpolypropylene, is of a substantially ellipsoidal basic shape with arecess 3 in which a fluid can collect, shown here containing capturedliquid 2. A gas stream to be treated is represented by the arrows 5,flowing upwards through the column.

A treatment liquid is introduced into the column from an inlet 6 in aninitial direction indicated by arrows 7. The vicinity around the element1 is shown in schematic form. The element 1 has a displacement volume of22.5 ml. The recess 3, measured relative to an equivalent ellipsoid withan upper envelope of the same shape as the illustrated lower envelope,has a volume of 5.7 ml, i.e. about 25% of the element volume. Thedensity of the element 1 with no captured liquid in the recess 3 is 220kg/m³, rising instantaneously to 260 kg/m³ when the recess 3 is filledto capacity with liquid 2.

FIG. 2 shows the tumbling action of the element 1. At point A theelement has its recess 3 uppermost. The element 1 then tumbles in adirection denoted by arrow 9 until it has turned through 180° to pointB, when the recess is lowermost.

When the recess 3 is at point A it gathers liquid from the down-flowingliquid 7 whilst being buffeted by the rising gas stream 5. The capturedliquid 2 causes an increase in the density of the upper part of theelement which increases the pressure gradient. The element 1 tumblesfrom position A, the extra weight of the liquid 2 on the descending sideof the element 1 increasing the tumble velocity.

The liquid 2 in the recess 3 is displaced by the movement and by point Bthe recess 3 has entirely emptied of liquid. Now at the bottom of theelement 1, the recess 3 acts as a lip for the rising gas stream 5 topush the element back rapidly towards its upright position A.

FIG. 3 shows a cross-sectional view of a second version of packingelement according to the invention. This packing element 11, also ofmoulded polypropylene, is of a substantially spherical basic shape buthas two opposing recesses 13 and 14. The recess 13 is shown containingcaptured liquid 12.

An upwardly-flowing gas stream to be treated is represented by the arrow15.

Treatment liquid passes downwards as indicated by the arrow 17.

The element 11 is thus simultaneously subjected to the effects ofcaptured liquid 12 in recess 13 and captured gas in recess 14. Tumblingof the element 11 in the direction indicated by arrows 18 is thusincreased by both these effects.

The FIG. 4 graph shows the performance (measured as the inverse of theheight of a transfer unit (HTU⁻¹, with units of m⁻¹), against thepressure gradient (ΔP/H_(o), with units of Pa/m) for elements in afluidised bed. The total packing height requirement is the product ofthe height of a transfer unit (HTU) and number of transfer units (NTU).The graph effectively compares the relative bed heights of the packingelements, including a perfect 2D (two-dimensional) ellipsoid (oblatespheroid), ovoidal ellipsoids such as ‘birds eggs’ and ‘acorns’ bothwith and without COG/COS offset, and an indented ‘acorn’ shape. Thelower bed heights represent better transfer efficiency.

By taking ratios of 1/HTU⁻¹ or HTU the relative bed heights are apparentand it can be seen that the lowest bed heights were most easilyattainable with the indented acorn (TurboPak®) element having a 29%COG/COS offset and an indentation of 5.6% of the basic element. The nextbest element in terms of the ΔP/NTU ratio was the 0% offset whole-acornshape marketed as Turboid®, although this element was much inferior interms of effective bed height (HTU⁻¹).

FIG. 5 shows the energy efficacy in a fluidised bed of the packingelements of FIG. 4 by comparing their pressure drop/efficiency (ΔP/NTU)with their pressure gradient (ΔP/H_(o)) measured in a system absorbingacetone at 20° C. It is clear that the 29% offset, 5.6% indented acornis at or near a minimum point in terms of energy required to achieve acertain absorption efficiency. It is better than the 0% offsetnon-indented 38×32 mm acorn and substantially better than the oblatespheroid and ovoidal ‘egg’ shapes tested. This shows that both bedheights and pressure drops are improved by the elements of the inventionover existing fluidisable packing elements.

Tests were undertaken to compare a 38 mm×32 mm 5 gram 29% indented‘acorn’ shape (TurboPak®) against a 57 mm×40 mm 7 gram ‘bird's egg’shape.

Whilst the latter larger element had the advantage of a longer semi-axisand 29% COG/COS offset to increase its tumbling rate this was to someextent balanced by the higher density of the non offset outline smallerelement.

With a gas flow velocity of 3.8 m/s and a liquid to gas ratio of 3litres/m³ in a system absorbing acetone at 20° C. there was a surprisingimprovement in performance of 125% over the ‘egg’ when measured in termsof mass or heat transfer coefficient (HTU⁻¹)

1. A packing element for use in a fluidised bed in which a gas streamand a liquid stream pass in counter-current directions to each other,comprising: a continuous surface with at least one recessed portion, theat least one recessed portion having a volume between 0.2 and 30% of avolume of the packing element, the packing element being arranged to andbeing shaped such as to capture at least one of liquid from the liquidstream and gas from the gas stream, wherein the packing element iscomposed of a hollow plastic material, the material being composed ofone of polypropylene and polyethylene and polyvinyl difluoride (PVDF).2. A packing element according to claim 1, wherein a basic outline shapeof the packing element is one of parallelepipedal, cylindrical, conical,frusto-conical, parabaloidal, star-shaped, spherical and ellipsoidal. 3.A packing element according to claim 1, wherein the basic shape isasymmetrical.
 4. A packing element according to claim 1, wherein thebasic shape is one of egg-shaped, ovoidal and acorn-shaped.
 5. A packingelement according to claim 1, wherein a shape of the at least onerecessed portion is one of straight-sided, concave and annular.
 6. Apacking element according to claim 1, wherein the packing element has alongest axis and the recessed portion is located at one end of thataxis.
 7. A packing element for use in a fluidised bed in which a gasstream and a liquid stream pass in counter-current directions to eachother, comprising: a continuous surface with at least one recessedportion, the at least one recessed portion having a volume between 0.2and 30% of a volume of the packing element, the packing element beingarranged to and being shaped such as to capture at least one of liquidfrom the liquid stream and gas from the gas stream, wherein a length ofa longest axis of the packing element is between 10 mm and 125 mm.
 8. Apacking element for use in a fluidised bed in which a gas stream and aliquid stream pass in counter-current directions to each other,comprising: a continuous surface with at least one recessed portion, theat least one recessed portion having a volume between 0.2 and 30% of avolume of the packing element, the packing element being arranged to andbeing shaped such as to capture at least one of liquid from the liquidstream and gas from the gas stream, wherein when the packing element hasa first shape, the at least one recessed portion includes a firstrecessed portion situated at a first end of the packing element, thefirst shape including one of a basic ellipsoidal shape and a shapesubstantially similar to the basic ellipsoidal shape and wherein whenthe packing element has a second shape, the at least one recessedportion includes the first recessed portion situated at the first end ofthe packing element and a second recessed portion situated at a secondend of the packing element, the first end being situated opposite thesecond, the second shape being any shape other than the first shape. 9.A packing element for use in a fluidised bed in which a gas stream and aliquid stream pass in counter-current directions to each other,comprising: a continuous surface with at least one recessed portion, theat least one recessed portion having a volume between 0.2 and 30% of avolume of the packing element, the packing element being arranged to andbeing shaped such as to capture at least one of liquid from the liquidstream and gas from the gas stream, wherein a volume of the at least onerecessed portion is between 1 and 15% of the volume of the packingelement.
 10. A packing element for use in a fluidised bed in which a gasstream and a liquid stream pass in counter-current directions to eachother, comprising: a continuous surface with at least one recessedportion, the at least one recessed portion having a volume between 0.2and 30% of a volume of the packing element, the packing element beingarranged to and being shaped such as to capture at least one of liquidfrom the liquid stream and gas from the gas stream, wherein a shapefactor is one of: (I) between 0.3 and 0.9 for an indented basic outlineshape and (ii) between 0.3 and 1.0 for a naturally indented shape.
 11. Apacking element for use in a fluidised bed in which a gas stream and aliquid stream pass in counter-current directions to each other,comprising: a continuous surface with at least one recessed portion, theat least one recessed portion having a volume between 0.2 and 30% of avolume of the packing element, the packing element being arranged to andbeing shaped such as to capture at least one of liquid from the liquidstream and gas from the gas stream, wherein an effective density of thepacking element is between 100 and 500 kg/m³.
 12. A method for at leastone of absorption, desorption, distillation, heat transfer, scrubbing,stripping and particulate transfer, in a column, the method comprisingthe step of: passing a gas stream and a liquid stream in directionscounter-current to each other through a fluidised bed which includes apacking element, the packing element including a continuous surface withat least one recessed portion having a volume between 0.2 and 30% of avolume of a packing element, and being shaped such as to capture atleast one of liquid from the liquid stream and gas from the gas stream,wherein a velocity of the gas upwards through the column is between 0.2and 10 m/s.
 13. A method for at least one of absorption desorption,distillation, beat transfer, scrubbing, stripping and particulatetransfer in a column, the method comprising the step of: passing a gasstream and a liquid stream in directions counter-current to each otherthrough a fluidised bed which includes a packing element, the packingelement including a continuous surface with at least one recessedportion having a volume between 0.2 and 30% of a volume of a packingelement, and being shaped such as to capture at least one of liquid fromthe liquid stream and gas from the gas stream, wherein a liquid to gasvolume ratio is between 0.1 and 50 dm³/m³.
 14. A method for at least oneof absorption, desorption, distillation, heat transfer, scrubbing,stripping and particulate transfer, in a column, the method comprisingthe step of: passing a gas stream and a liquid stream in directionscounter-current to each other through a fluidised bed which includes apacking element, the packing element including a continuous surface withat least one recessed portion having a volume between 0.2 and 30% of avolume of a packing element, and being shaped such as to capture atleast one of liquid from the liquid stream and gas from the gas stream,wherein the liquid stream is falling and capture of falling liquid in arecessed portion of the packing element causes a change in the effectivedensity of the packing element.
 15. A method for at least one ofabsorption, desorption, distillation, heat transfer, scrubbing,stripping and particulate transfer, in a column, the method comprisingthe step of: passing a gas stream and a liquid stream in directionscounter-current to each other through a fluidised bed which includes apacking element, the packing element including a continuous surface withat least one recessed portion having a volume between 0.2 and 30% of avolume of a packing element, and being shared such as to capture atleast one of liquid from the liquid stream and gas from the gas stream,wherein the gas stream is rising and the capture of rising gas in arecessed portion of the packing element causes a change in the effectivedensity of the packing element.
 16. A method for at least one ofabsorption, desorption, distillation, heat transfer, scrubbing,stripping and particulate transfer, in a column, the method comprisingthe step of: passing a gas stream and a liquid stream in directionscounter-current to each other through a fluidised bed which includes apacking element, the packing element including a continuous surface withat least one recessed portion having a volume between 0.2 and 30% of avolume of a packing element, and being shaped such as to capture atleast one of liquid from the liquid stream and gas from the gas stream,wherein the liquid stream is falling and the capture of falling liquidin a recessed portion of the packing element causes a change in thecentre of gravity of the packing element.
 17. A method for at least oneof absorption, desorption, distillation, heat transfer, scrubbing,stripping and particulate transfer, in a column, the method comprisingthe step of: passing a gas stream and a liquid stream in directionscounter-current to each other through a fluidised bed which includes apacking element, the packing element including a continuous surface withat least one recessed portion having a volume between 0.2 and 30% of avolume of a packing element, and being shaped such as to capture atleast one of liquid from the liquid stream and gas from the gas stream,wherein the gas stream is rising and the capture of rising gas in arecessed portion of the packing element causes a change in the centre ofgravity of the packing element.
 18. A method for at least one ofabsorption, desorption, distillation, heat transfer, scrubbing,stripping and particulate transfer, in a column, the method comprisingthe step of: passing a gas stream and a liquid stream in directionscounter-current to each other through a fluidised bed which includes apacking element, the packing element including a continuous surface withat least one recessed portion having a volume between 0.2 and 30% of avolume of a packing element, and being shaped such as to capture atleast one of liquid from the liquid stream and gas from the gas stream,wherein a change in the centre of gravity of the packing element causethe packing element to tumble.
 19. A method for at least one ofabsorption, desorption, distillation, heat transfer, scrubbing,stripping and particulate transfer, in a column, the method comprisingthe step of: passing a gas stream and a liquid stream in directionscounter-current to each other through a fluidised bed which includes apacking element, the packing element including a continuous surface withat least one recessed portion having a volume between 0.2 and 30% of avolume of a packing element, and being shaped such as to capture atleast one of liquid from the liquid stream and gas from the gas stream,wherein the liquid stream is falling and the capture of falling liquidin the recessed portion of the packing element causes the packingelement to tumble.
 20. A method as claimed in claim 19, wherein thecaptured liquid is released as the packing element tumbles.
 21. A methodfor at least one of absorption, desorption, distillation, heat transfer,scrubbing stripping and particulate transfer, in a column, the methodcomprising the step of: passing a gas stream and a liquid stream indirections counter-current to each other through a fluidised bed whichincludes a packing element, the packing element including a continuoussurface with at least one recessed portion having a volume between 0.2and 30% of a volume of a packing element, and being shaped such as tocapture at least one of liquid from the liquid stream and gas from thegas stream, wherein the gas stream is rising and the capture of risinggas in the recessed portion of the packing element causes the packingelement to tumble.
 22. A method as claimed in claim 21, wherein thecaptured gas is released as the packing element tumbles.
 23. A methodfor at least one of absorption desorption distillation, heat transfer,scrubbing, stripping and particulate transfer, in a column, the methodcomprising the step of: passing a gas, stream and a liquid stream indirections counter-current to each other through a fluidised bed whichincludes a packing element, the packing element including a continuoussurface with at least one recessed portion having a volume between 0.2and 30% of a volume of a packing element, and being shaped such as tocapture at least one of liquid from the liquid stream and gas from thegas stream, wherein the liquid stream is falling and the gas stream isrising and the capture of falling liquid and rising gas in the recessedportion of the packing element causes the packing element to tumble. 24.A method for at least one of absorption, desorption, distillation, heattransfer, scrubbing, stripping and particulate transfer, in a column,the method comprising the step of: passing a gas stream and a liquidstream in directions counter-current to each other through a fluidisedbed which includes a packing element, the packing element including acontinuous surface with at least one recessed portion having a volumebetween 0.2 and 30% of a volume of a packing element, and being shapedsuch as to capture at least one of liquid from the liquid stream and gasfrom the gas stream, wherein the liquid stream is falling and the gasstream is rising and the capture of at least one of falling liquid orrising gas causes the packing element to tumble, the tumbling motioncausing turbulence in the gas and liquid streams.